The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Internal combustion engines, especially automotive internal combustion engines, generally fall into one of two categories, spark ignition engines and compression ignition engines. Traditional spark ignition engines, such as gasoline engines, typically function by introducing a fuel/air mixture into the combustion cylinders, which is then compressed in the compression stroke and ignited by a spark plug. Traditional compression ignition engines, such as diesel engines, typically function by introducing or injecting pressurized fuel into a combustion cylinder near top dead center (TDC) of the compression stroke, which ignites upon injection. Combustion for both traditional gasoline engines and diesel engines involves premixed or diffusion flames that are controlled by fluid mechanics. Each type of engine has advantages and disadvantages. In general, gasoline engines produce fewer emissions but are less efficient, while, in general, diesel engines are more efficient but produce more emissions.
More recently, other types of combustion methodologies have been introduced for internal combustion engines. One of these combustion concepts is known in the art as controlled auto-ignition, or homogeneous charge compression ignition (HCCI). Controlled auto-ignition comprises a distributed, flameless, auto-ignition combustion process that is controlled by oxidation chemistry, rather than by fluid mechanics. In a typical HCCI engine, the cylinder charge is nearly homogeneous in composition, temperature, and residual level at intake valve closing time. Because auto-ignition combustion is a distributed kinetically-controlled combustion process, an HCCI engine operates with a dilute fuel/air mixture (i.e., lean of a fuel/air stoichiometric point) and has a relatively low peak combustion temperature, thus forming extremely low NOx emissions. The fuel/air mixture for auto-ignition is relatively homogeneous, as compared to the stratified fuel/air combustion mixtures used in diesel engines, and, therefore, the rich zones that form smoke and particulate emissions in diesel engines are substantially eliminated. Because of this dilute fuel/air mixture, a HCCI engine can operate unthrottled to achieve diesel-like fuel economy.
At medium engine speed and load, a combination of valve profile and timing (e.g., exhaust recompression and exhaust re-breathing) and fueling strategy has been found to be effective in providing adequate heating to the cylinder charge so that auto-ignition during the compression stroke leads to stable combustion with low noise. One of the main issues in effectively operating an HCCI engine has been to control the combustion process properly so that robust and stable combustion resulting in low emissions, optimal heat release rate, and low noise can be achieved over a range of operating conditions. The benefits of auto-ignition combustion have been known for many years. The primary barrier to product implementation, however, has been the inability to control the auto-ignition combustion process.
To address issues related to combustion stability, HCCI engines operate at different combustion modes, depending upon specific engine operating conditions. The different combustion modes include various spark-ignition modes and auto-ignition modes.
The combustion process in an HCCI engine depends strongly on factors such as cylinder charge composition, temperature, and pressure at the intake valve closing. Hence, the control inputs to the engine, for example, fuel mass and injection timing and intake/exhaust valve profile, must be carefully coordinated to ensure robust auto-ignition combustion. Generally speaking, for best fuel economy, an HCCI engine operates unthrottled and with a lean air-fuel mixture. Further, in an HCCI engine using exhaust recompression valve strategy, the cylinder charge temperature is controlled by trapping different amount of the hot residual gas from the previous cycle by varying the exhaust valve close timing. The opening timing of the intake valve is delayed than normal to a later time preferably symmetrical to the exhaust valve closing timing about top-dead-center (TDC) intake. Both the cylinder charge composition and temperature are strongly affected by the exhaust valve closing timing. In particular, more hot residual gas from a previous cycle is retained with earlier closing of the exhaust valve which leaves less room for incoming fresh air mass. The net effects are higher cylinder charge temperature and lower cylinder oxygen concentration.
For a single cylinder engine, it has been demonstrated that by adjusting both intake/exhaust valve profiles and engine control inputs such as injection mass and timing, spark timing, throttle and EGR valve positions combustion phasing control and robust auto-ignition combustion can be achieved using either a fully flexible valve actuation (FFVA) system or a mechanical two-step variable valve lift control scheme with a dual cam phasing system. However, in a multi-cylinder HCCI engine, combustion in each cylinder can vary significantly due to the difference in temperature caused by air, EGR and thermal mal-distributions. To compensate for such variations in cylinders and to stabilize the auto-ignited combustion, fuel quantity at each individual cylinder may be controlled.
In an HCCI engine, temperature at intake valve closing at each cylinder is critical since it determines the stability of combustion especially during transients. During transients, if the temperature at intake valve closing is too low at a particular cylinder, either misfire or partial burn, which may cause undesired drivability problems can occur at that cylinder. The combustion related parameters measured during transients are reliable indicators if the temperature at intake valve closing at a particular cylinder is too low.
A system which detects the cases wherein the temperature at intake valve closing is too low is now described.